Considering specs, price, and availability, i'd say the Spectre 360 is the best competitor for the Surface Book.

But, in my opinion, the detachable display is a significant difference maker between the two devices. I'm currently a Surface Pro 3 user, but i'd like to move to a Surface Book (or Surface Book 2) in the near future. I detach the display on my Surface Pro 3 all the time. For media consumption, for passing it around during a meeting, and for teaching from.

If i'm using a 2-in-1 device as a tablet, i don't want to have to have the keyboard attached to it.

So for that reasoning alone, i'd still stick to a Surface Book over an HP Spectre.